Cysteine proteinases contain a catalytically active cysteine sulfhydryl group and a histidine imidazole group within the active site of the enzyme. The imidazole group polarizes and deprotonates the Cysteine sulfhydryl group. Cysteine proteinases show unusual reactivity toward electrophiles relative to the nucleophilicity of sulfur in cysteine, although each cysteine proteinase appears to hydrolyze different proteins and peptide substrates at different rates. Among the members of this enzyme class are papain, cathepsin B, cathepsin H, cathepsin L, and hepatitis A viral protease. Selective inhibitors of the cysteine proteinase cathepsin B have potential therapeutic value in disease states such as proteinuria in glomerular disease, osteoclastic bone resorption, tumor metastasis, tissue damage in myocardial infarction and muscle wasting in Duchenne muscular dystrophy. The cathepsins are implicated in tumor invasion also.
Unlike the aspartic and zinc proteases, the cysteine proteases do not catalyze the direct addition of water to the amide carbonyl group, but rather the acyl moiety is transferred first to the active site thiol (e.g. Cys-25 in papain), and the thioester is cleaved in a second step. Two tetrahedral intermediates are thereby involved, both of which are covalently linked to the enzyme.
A variety of low molecular weight inhibitors of cysteine proteinases known as of 1984 selectively to inhibit such enzymes are described by Rich, Proteinase Inhibitors (Chapter 4, "Inhibitors of Cysteine Proteinases"), Elsevier Science Publishers (1986). One group of cysteine proteinase inhibitors are peptide aldehydes. The peptidyl aldehydes combine with the cysteine thiol to form a thiohemiacetal in the active site. These aldehyde inhibitors are generally analogs of the transition-state. However, inhibitors of the aldehyde class are not very stable in vivo (degrade very quickly) and are also not very shelf-life stable. Although other classes of inhibitors are known (such as peptidyl chloromethanes and peptidyl diazomethanes), these have varying degrees of affinity for selectivity and often have some non-specific activity that raises potential problems of side effects.
Krantz et al., Biochemistry, 30, pp. 4678-4687 (1991) and Krantz et al., U.S. Pat. No. 5,055,451, issued Oct. 8, 1991, describe aryloxy and arylacyloxy methyl ketones as thiol protease inhibitors, and particularly certain inhibitors of Cathepsin B. Shaw, Adv. Enzymol. Relat. Areas Mol. Biol., 63, 271 (990) describes peptidyl methyl ketones with a leaving group that can contain --CHS(CH.sub.3).sub.2 in designing inactivators of cysteine proteinases. However, the Shaw sulfur moieties are part of the leaving groups themselves.